Thermal detectors are detectors that operate by absorbing energy from electromagnetic radiation incident thereon and by converting the heat thus generated into an electrical signal representative of the amount of absorbed radiation. Perhaps the most prominent type of thermal detectors currently available is microbolometer detectors, usually shortened as microbolometers. A microbolometer is typically based on a suspended platform or bridge structure having a low thermal mass and on which is disposed a material having a temperature-dependent electrical resistance. The platform is generally held above and thermally insulated from a substrate by a support structure, and is provided with a thermistor, which is the resistive element whose electrical resistance changes in response to temperature variations caused by the absorbed radiation. The thermistor may, for example, be composed of a material having a high temperature coefficient of resistance (TCR) such as vanadium oxide.
The support structure generally includes support posts and arms (or legs) that provide mechanical stability and thermal insulation to the platform, as well as electrical connectivity between the thermistor and a readout electrical circuit provided in the substrate and containing circuitry for measuring changes in the resistance of the thermistor. Furthermore, in most current microbolometer designs the support structure (i.e. the support posts and arms) is generally distributed along an outer perimeter of the platform, while the thermistor is located in a central region of the platform, inwardly of the support structure. Such an arrangement of the support structure may be referred to herein as an “edge support” or an “edge post” configuration.
Arrays of microbolometer detectors may be fabricated on a substrate using common and relatively inexpensive integrated circuit fabrication techniques. Such arrays are often referred to as focal plane arrays (FPAs). They are capable of operating at room temperature without requiring cryogenic cooling, and can be used in a wide variety of applications, including night vision, observation of the Earth from space, pollution and fire detection, spectroscopy, and border control.
In most current applications, arrays of microbolometers are used to sense radiation in the infrared portion of the electromagnetic spectrum, usually in the mid-wave infrared, encompassing wavelengths of between about 3 and 5 μm (micrometers), or in the long-wave infrared, encompassing wavelengths of between about 8 and 14 μm. Arrays of microbolometer detectors are often integrated in uncooled thermal cameras for sensing incoming infrared radiation from a target scene. Each microbolometer detector of the array absorbs some infrared radiation resulting in a corresponding change in the microbolometer detector temperature, which produces a corresponding change in electrical resistance. A two-dimensional pixelated thermal image representative of the infrared radiation incident from the scene can be generated by converting the changes in electrical resistance of each microbolometer detector of the array into an electrical signal that can be displayed on a screen or stored for later viewing or processing. State-of-the-art arrays of infrared microbolometer detectors now include 1024 by 768 pixel arrays with a 17-μm pixel pitch.
In the last decade, there has been a growing interest toward extending microbolometer spectroscopy and sensing applications beyond the traditional infrared range, namely in the far-infrared and terahertz (or sub-millimeter) spectral regions. As known in the art, these portions of the electromagnetic spectrum have long been relatively unused for industrial purposes due to the lack of efficient techniques for detection and generation of radiation in this frequency range. However, in recent years, considerable attention has been devoted to the development of high-power sources for terahertz applications and to the improvement of the sensitivity of terahertz detectors. In particular, much research has been carried out for optimizing the sensitivity of microbolometer detectors and arrays thereof, notably by relying on various metallic and organic absorbers, antenna-coupled detectors, and frequency selective surfaces.
In many terahertz applications, the sensitivity of a detector is more important than the spatial resolution thereof. As a result, arrays of microbolometers having a relatively large pixel area, corresponding to a relatively large platform, are often preferable to achieve an efficient collection of radiation while providing an improved signal to noise ratio. For example, some microbolometers specifically designed for terahertz applications may have a platform with a surface area as large as 200 by 200 μm2, which is considerably larger than that of their counterparts intended primarily for infrared imaging. However, increasing the size of the platform presents many design challenges, in particular related to the overall mechanical stability of the microbolometer detector.
There thus exists a need in the art for a microbolometer detector with a large platform suited for far-infrared and terahertz applications.